Double Cream Dilution
Horses homozygous for the Cream Dilution gene (double cream) are characterized by light-beige color, varying from pale cream or almost white to saturated yellow (Photo 24). The skin is pink and eyelashes yellow or reddish. The eyes are most often blue, although you can also find golden, green, and greenish-blue with amber specks. In old literature on the subject, this color was not differentiated from light palomino (see p. 19).
In the past double cream diluted color was considered the manifestation of albinism, but this is incorrect. Albinism assumes the absence of any pigment in the hair and skin. White marks on the nose and legs are clearly visible in double cream diluted horses, consistent with the presence of pigment. The mane and tail could be the same color as the body, but it can also be brighter (white), or darker. When present, dapples on these horses have a light coffee color. The intensity of the color can be of an average shade, or light or dark.
Regardless of their base color, double cream horses all have the same name in the Russian language: “Isabell.” However, there are distinctive terms used in the United States and elsewhere, depending on the base colors: cremello (chestnut base), perlino (bay base), and smoky cream (black base). Barbara Kostelnik (2008) uses brown cream for homozygous cream combined with the seal brown base. In reality, all these terms are names based on genotypes. In the United States the opinion exists that it is possible to make a conclusion about the genotype of a horse from the shade of the color. It is believed that in cremello horses, the mane and tail can be white, pale cream, or reddish; in perlino the mane and tail are light brown or ashy, as well as the lower part of the legs; and in smoky cream there is a light ashy tone of the body, mane, and tail. However, studies have not revealed a precise correlation between these nuances of hair color and the genotype of double cream horses.
Foals are born very light, almost white, with pink skin and light blue eyes. Sometimes, when grown a little more, the foal’s coat can have a light golden tinge. Double cream horses are vulnerable to the sun and frequently suffer from burns on the nose, and sometimes also from skin cancer. Their eyes are sensitive to the bright sunlight. Long sun exposure can cause their skin to become spotty, as they develop reddish pigmented areas (so-called “pumpkin color”), and sometimes can even become gray, especially around the eyes. Because of the sensitivity to sun, this color was actively selected against by breeders in different parts of the world, Australia or Turkmenistan, for example.
Double cream dilute colors are relatively rare, and as might be expected are found in breeds in which other variants of Cream Dilution occur (palomino, buckskin, smoky black). Examples include the Lusitano, American Saddlebred, Tennessee Walker, Icelandic Horse, Miniature Horse, Paso Fino, Peruvian Paso, Quarter Horse, Welsh Pony, Australian Pony, Shetland Pony, Kinsky, Akhal-Teke, and Byelorussian Harness Horse.
Inheritance of Colors Determined by Cream Dilution
The Cream Dilution mutation is located in the MATP (membrane-associated transporter protein) gene on the twenty-first chromosome. This gene is responsible for the synthesis of a protein, which participates in the process of pigment formation and melanocyte differentiation. This protein is also found in large quantities in melanoma tumor cells.
The mutation of MATP is designated “Ccr,” while the “Wild” allele that does not influence the color of the animal is designated by “C” or “Cr.” The “Ccr” allele is incompletely dominant, and in the heterozygous state it causes dilution of pheomelanin-pigmented regions without an effect on eumelanin colored areas. However, observations show that the regions pigmented with eumelanin also show the action of this gene, although it is not always clearly noticeable. An example can be the brownish coloring of the lower part of the legs in some buckskins, or the brownish shades sometimes encountered in smoky black horses. In some cases the “Ccr” gene in the heterozygous state lightens up the iris color in the eye, which then becomes hazel or amber. A horse homozygous for “Ccr” shows strong pigment dilution not only in the hair, but also in the skin and the iris of the eye (Photo 25). The colors caused by Cream Dilution are summarized in Table 3 (see p. 22).
A DNA test for “Ccr” is available, and it is especially useful to determine the smoky black color, which as mentioned earlier, in the majority of cases cannot be distinguished visually from black. Based on the results of this test, the breeder will know if a particular horse can produce certain diluted colors caused by this mutation, and plan appropriate breeding. Tests of Agouti and Extension further help to determine the precise genotypes in double cream dilutes.
Table 3.
| Base Color | Ccr/C | Ccr/Ccr |
|---|---|---|
| Bay | Buckskin | Perlino |
| Black | Smoky Black | Smoky Cream |
| Chestnut | Palomino | Cremello |
| Seal Brown | Smoky Brown | Perlino |
COLORS DETERMINED BY THE DUN GENE
“Primitive” Markings
The color determined by the Dun gene (Dn+ allele) in combination with the bay base color is genetically close to the color of the ancestors of the domestic horse, as well as contemporary wild representatives of the species. They are also typical for aboriginal species. Including this group of colors in the category of dilutions is only relative: The main characteristic of the color dun is the presence of the so-called “Wild” or “primitive” markings. The dilution of hair is only an additional feature, and it is not present or obvious in all cases.
The following primitive markings are seen in horses with the Dun gene:
The dorsal stripe is a dark stripe that runs along the horse’s spine from the withers to the dock of the tail. Its main characteristic is clearly defined boundaries (Photos 26 & 27). It is possible to liken it to a wide line drawn on the horse with a fat-tipped marker. Sometimes you can also see short transverse stripes, which is called fishboning (or barbs) due to the similarity with a fish’s spine. Occasionally in horses with strong color dilution you can also find a zigzag-shaped or discontinuous dorsal stripe.
Zebra bars or zebra stripes are short, dark, transverse stripes on the horse’s legs, located in or above the region of the knees and hocks (Photo 28). Zebra bars are frequently present only on the backside of the legs.
Often on the backside of the lower part of the horse’s legs, you will see a line of lighter hair, known by some as a zipper (see Photo 109).
Cobwebbing or lacing is a net of dark lines, which converge in the center of forehead of some dun horses, resembling a